1. Field of the Invention
The present invention relates to a phase modulation holographic device for use in a multi focal length holographic lens system and a holographic coupler.
Particularly, the present invention can be employed as a zone plate for simultaneously generating a plurality of wave-fronts in a shape measuring apparatus as described in Japanese Patent Application H4-168825 of the present Applicants.
2. Description of the Prior Art
As shown in FIG. 1, a typical holographic device 3 is constituted by a phase modulation holographic device 2 and an amplitude modulation holographic device 1. As the phase modulation holographic device 2, a phase plate called a kinoform has been known. This is a holographic device for providing to a plane wave a phase modulation which is necessary for reconstituting an arbitrary image. For example, by providing saw-toothed convex portions 2a as shown in FIG. 1, an uneven surface is formed with which the phase modulation is performed.
The amplitude modulation holographic device 1 is a plate-like device where portions la having a small intensity distribution and through which it is difficult for light to pass and portions lb having a large intensity distribution and through which it is easy for light to pass are alternately arranged. The amplitude modulation holographic device 1 and the phase modulation holographic device 2 are joined together to form one optical device (i.e. the holographic device 3).
The amplitude modulation holographic device 1 and the phase modulation holographic device 2 can easily be produced. However, since highly accurate positioning is required in the process of joining the two devices, various problems are presented such as difficulty in the joining of the two devices, insufficient precision and high cost.